Well. Statistics, practice and probability.
In Base jumping its just you and the parachute. The parachute is packed and tested and you only jump above safe release heights. As soon as you jump you throw the drag chute which pulls out your main chute in seconds. You then float to the ground to a designated safe landing area. The only danger is crosswinds and poor control by the "parachuter"
In a glider your height that you "eject" is not definite and predetermined. It occurs only in an emergency. So it could be a low altitude and zero speed. You then have to unharness, pop the canopy, and jump. Then release the parachute and try to find somewhere safe to land (unlike basejumping, you have no pre-detremined landing area.)
So with base jumping:
You know your height.
You know your landing area.
you know your wind speeds.
You know your hazzards.
With a Glider:
You don't.
